While pirates usually attack cargo ships, they have hijacked a number of yachts in recent years.

Ecoterra said the capture of the S/V Quest had been reported by both its sources and by Nato's anti-piracy operation, Ocean Shield. Nato could not be reached immediately for comment.

Jean and Scott Adam, the yacht's owners, have been sailing it around the world since 2002, according to their website.

The couple wrote on the site that they had taken on two new crew members last year.

Mapping out their sailing plans for this year, they said they planned to sail from Sri Lanka to Crete in the Mediterranean, via the Suez Canal, making stops in India, Oman and Djibouti.

Vulnerable shipping

Somalia's UN mission confirmed for the Associated Press news agency in New York that the S/V Quest had been hijacked.

Omar Jamal, first secretary at the mission, called for the immediate release of the hostages and all other captives who are in the hands of the pirates.

Overstretched international anti-piracy forces operating in the Indian Ocean give priority to protecting cargo ships. The EU's Navfor force recently warned that yachts, even those travelling in convoys, were not assured of protection.

Individual yacht owners wishing to reach the Suez Canal through the Gulf of Aden are tempted, as a result, to leave convoys and strike out by themselves, maritime experts say.

The attack on the S/V Quest is the latest in a number of attacks on yachts:

South African couple Bruno Pelizzari and Deborah Calitz are still being held in Somalia, four months after their vessel, the S/Y Choizil, was hijacked

British couple Paul and Rachel Chandler were held for nearly 400 days after their yacht, the Lynn Rival, was hijacked near the Seychelles in October 2009

French yacht-owner Florent Lemacon was killed in April 2009 when French commandos tried to liberate him and four other people from their hijacked yacht, the Tanit, off Somalia

Somalia has had no functioning central government since 1991, allowing piracy to flourish off its coast.

Somali pirates have made millions of dollars in recent years by capturing cargo vessels in the shipping lanes around the Horn of Africa and holding the ships and crew for ransom.

A recent US study found that maritime piracy costs the global economy between $7bn (£4.4bn) and $12bn (£7.6bn) a year.